Microstructural evolution and characterization of ground granulated blast furnace slag in variant pH

Abstract

In order to explore the difference of the activation effect on the ground granulated blast furnace slag (GGBFS) subjected to variant activators as well as to further clarify the depolymerization mechanism of GGBFS, investigation on the dissolved mechanism, hydrated characteristics and microstructural evolution were carried out based on slag immersed in solutions with variant pH, and FTIR, NMR, XRD, SEM and EDS were used to analyze structure of GGBFS, the change of valence and bond, structural coordination, hydrated products, micromorphology and elements distribution. The results show that the H+ destroyed the structure of GGBFS, and a small amount of akermanite and gehlenite particles were formed on the surface of slag, Ca2+, Mg2+ and Al(OH)2+ ions were released as 3 ≤ pH < 7. For the pH < 3, cauliflower-like chabazite particles were accumulated, and the dissolved Si and Al gradually formed monomeric siloxane and hydrated aluminium complex ion [Al(H2O)6]3+, respectively. The H+ ionized by the water eroded the slag surface and corrugated melilite particles, together with a layer of Si(OH)4 and Al(OH)3 gel attached on the slag surface for pH = 7. As 7 < pH ≤ 12, the hydrated magnesium aluminate and calcite particles were generated on the slag surface, and Ca2+, Mg2+ and Al(OH)4− ions were released under the influence of OH−. The Si-O-Al bond was destructed and the monomers of Al(OH)4- and Si(OH)4 were released for pH > 12. The Si(OH)4 and Al(OH)4− performed polycondensation in the presence of the cations such as Ca2+, and that finally led to formation of large number C-(A)-S-H with a corrugated morphology.